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Mysterious bright patches on asteroids explained

The relatively dark Sagamihara region of asteroid Itokawa appears to be older and coated with more red iron dust than the younger bright patch (inside square) in the Tsukuba region

(Image: JAXA)

By David Shiga

Asteroids gradually become coated with iron dust in space, becoming darker and redder with time, close-up observations of the asteroid Itokawa suggest. This confirms long-held suspicions about why asteroids look different than the space rocks that land on Earth as meteorites.

Chunks of asteroids that were broken off during collisions are thought to make up the most common kind of meteorite found on Earth – called ordinary chondrites. But strangely, the light spectra of these meteorites look different from the spectra of the most common type of asteroid, called S-type asteroids.

Some scientists have suggested that this is because the S-type asteroids have been “weathered” in space. In this process, fast-moving dust grains or energetic particles from the Sun could vaporise chunks of iron in the asteroid. The resulting cloud of iron particles would then rain back down on the space rock, leaving a reddish coating on its surface. Over time, the surface would become darker and redder.

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Observations of meteorites support the idea. When they are cut open, their interiors look fresh and relatively light in colour, presumably because they have never undergone any space weathering.

Colour variations

Earth-based observations have also shown that families of older asteroids – whose ages are calculated by tracing the orbits of their individual members backwards in time to determine when they broke apart from a single, parent body – tend to be darker and redder than younger space rocks.

But astronomers had never found clear evidence of another expected effect – that an asteroid would show variations in space weathering across its surface because collisions with other space rocks would expose fresh, unweathered material in some places.

Now, observations of asteroid 25413 Itokawa by Japan’s Hayabusa spacecraft have unambiguously found such variations. Researchers led by Takahiro Hiroi of Brown University in Providence, Rhode Island, US, analysed the spectra of infrared light from the 500-metre-long space rock, which appears to be an S-type asteroid.

The spectra look like what astronomers would see if a type of ordinary chondrite meteorite called an LL chondrite were coated with fine iron dust.

Meteorite strike

The researchers compared a relatively dark patch on the asteroid’s surface with one that is significantly lighter. The darker patch appears to contain more than twice as much iron dust than the lighter patch, suggesting that it has undergone more space weathering, the researchers say.

“Until now we haven’t had these kinds of close-up images and actual numbers on space weathering,” Hiroi told New Scientist.

He says the relatively unweathered material in the lighter patch may have been exposed more recently when a meteorite struck the asteroid.

Such an impact could expose fresh material by triggering a landslide or by blasting rocks off into space, he says. Itokawa’s low surface gravity means that anything moving at more than about 10 centimetres per second can escape the asteroid forever.

The Hayabusa results show that space weathering occurs as expected from artificial weathering experiments on Earth, says William Bottke of the Southwest Research Institute in Boulder, Colorado, US, who is not a member of Hiroi’s research team.

Space camouflage

“It suggests we’re really getting a handle on this problem,” he told New Scientist. “If you put something in space long enough, it can be camouflaged.”

Richard Binzel of the Massachusetts Institute of Technology in Cambridge, US, agrees. “The space weathering idea clears another hurdle” with Hayabusa’s observations of bright and dark patches, he told New Scientist.

Hayabusa was launched by the Japan Aerospace Exploration Agency (JAXA) in 2003 and reached Itokawa in September 2005. It was supposed to return a sample of the asteroid to Earth, but the sample collection attempt appears to have failed.

Still, a small amount of material from the asteroid may have made it into the collector by chance, and mission controllers hope to return the spacecraft to Earth by 2010 using only two or three of its four ion engines (see Hayabusa does have enough power to fly home).